Cathode ray tube display systems

ABSTRACT

A display system, having particular application in gamma cameras, in which items of information are represented by bright spots on the screen of a cathode ray tube. The display is basically in the form of a digital array of positions corresponding to different kinds of item, but is made easier to interpret by spreading out the spots for each position of the array.

O United States Patent 1 1 3,783,334 Gore et al. Jan. 1, 1974 [54]CATHODE RAY TUBE DISPLAY SYSTEMS 3,405,233 10/1968 Anger 250/71.5 S X3,440,480 4/1969 Henderson.. 315/18 [75] Inventors George Newbury;3,519,876 7/1970 Murray 315/22 x y Pel'flval Parker, Leatherhead-3,532,927 10/1970 Hindel 315/18 both of England A l R h D 1 OTHERPUBLICATIONS [73] ssignee: Nationa esearc eve opment Corporation,London, England ggnglgr 5e; al., Nucleomcs, V01. 21, No. 10, Oct. 1963,[22] Filed: July 1970 Anger, Nucleonics, Vol. 21, No. 10, Oct. 1963, pp.211 App]. No.: 55,495

Primary ExaminerLeland A. Sebastian [30] Forelgn Apphcat'on pnonty D ataAttorney-Cushman, Darby and Cushman July 23, 1969 Great Britain37,138/69 52 us. c1. 315/18, 250/71.5 s, 315/22, [57] ABSTRACT 315/30 Adisplay system, having particular application in [51] Int. Cl. H0lj29/80 gamma Cameras, in which items f information are [58] Field ofSearch 250/71.5 s; 315/18, represented y bright spots on the screen of acathode 315/22 ray tube. The display is basically in the form of adigital array of positions corresponding to different kinds [56] Referene Cit d of item, but is made easier to interpret by spreading UNITEDSTATES PATENTS out the spots for each position of the array.

2,942,109 6/1960 Bell et a]. 250/71.5 S 7 Claims, 3 Drawing Figures"(Juno PATENTED JAN 1 74 SHEET 2 BF 2 CATHODE RAY TUBE DISPLAY SYSTEMSThis invention relates to cathode ray tube display systems of the kindcomprising a cathode ray tube having a luminescent screen, means formodulating the electron beam of the tube by means of a series of uniformbrightening pulses which excite the screen to luminescence, and meansfor deflecting the electron beam by means of deflection pulsessynchronous with the brightening pulses, the brightening pulsesrepresenting individual items of information of different kinds and thedeflection pulses having amplitudes chosen so that the position at whichthe screen is excited by a brightening pulse depends upon the kind ofitem represented by that pulse.

Display systems of this kind may be used to provide a pictorialrepresentation of the relative frequencies of occurrence of events ofdifferent kinds, one application of particular interest being theprovision of twodimensional displays for gamma cameras. In thisapplication the brightening pulses correspond to the detection of gammarays emitted from an area of interest, for example an area of a humanbody into which an appropriate radio-isotope has been introduced for thepurpose of medical diagnosis; the deflection pulses have amplitudesselected so that gamma rays emitted from different sub-divisions of thearea will result in bright spots on the screen of the cathode ray tubeat different positions corresponding in layout to those of thesubdivisions. Those positions on the screen corresponding to thesub-divisions emitting the most rays will therefore be the mostfrequently illuminated, and if a photograph is taken of the screen usingan exposure which is long relative to the count-rate, or if a storagetube is used, the relative concentration of radio-activity over the areawill be represented by the variations in brightness over the photographor screen. Such a representation is known as a grey-scale display.

For ease of data handling, e.g. by a computer, it is desirable that thelocations of the sub-divisions should be characterised by digitalnumbers. If, in a display of the above kind, digital-valued deflectionpulses are used to produce the illuminated screen positions, the resultwill be a pattern of superimposed spots. The resulting photographs willtherefore show a pattern, e.g. a rectangular array, of light spots on adark background, the brightness of each spot being a measure of thenumber of superimposed spots and hence, in the aforementioned cameras,of the activity of the corresponding subdivision. Such a pattern isdifficult to interpret by the eye however, and in particular the abilityto recog- -nise relatively small differences in brightness betweenadjacent spots is limited.

It is an object of the present invention to provide a cathode ray tubedisplay system of the kind specified which is improved in this respect.

According to the invention, the deflection pulses are in the form ofdigital-valued pulses modulated in amplitude, the digital values beingselected according to the kinds of item represented by the brighteningpulses and corresponding to an array of discrete positions on thescreen, and the modulation being such that for each such position theexcitation of the screen is distributed over an appreciable portion ofthe screen.

The modulation may for example suitably be made such that the excitedportions of the screen corresponding to adjacent positions of the arrayabut each other or even overlap.

Preferably the rate-of-change of the modulation is made small relativeto the duration of the pulses, so as to avoid unduly broadening theindividual bright spots on the screen.

The modulation may be in the form of noise or may be a regularlyrecurrent signal with a sawtooth or other suitable waveform. Wheresubstantially Gaussian noise modulation is used, the modulation may beadjustable so that the excited portions of the screen corresponding toadjacent positions of the array overlap with half peak noise amplitudecorresponding to a point approximately midway between the adjacentpositions. Where noise modulation is used, the above-mentionedrate-ofchange condition can be achieved by limiting the highfrequencycontent of the noise.

The array of positions may be two-dimensional, forming for example arectangular array, but the invention is also applicable toone-dimensional arrays, for example, a single line of positions.

To enable the nature of the present invention to be more readilyunderstood, attention is directed, by way of example, to theaccompanying drawings wherein:

FIG. 1 is a block schematic diagram of the circuit of a gamma cameraincluding a display system according to the present invention,

FIG. 2 is an enlarged view of the form of grey-scale display obtainablewith the arrangement of FIG. 1,

FIG. 3 is a graph illustrating a preferred adjustment of modulatingamplitude using Gaussian noise.

Referring to FIG. 1, the gamma camera includes a radiation detector ofthe kind disclosed in copending U.S. Pat. application No. 831,639, filedJune 9, 1969, now U.S. Pat. No. 3,691,389, which comprises a rectangularslab of semiconductor material whose opposite faces are each providedwith eight parallel ribs 1 and 2, the ribs on one face being arranged atright angles to those on the other face. Each intersection of these ribsforms a discrete gamma ray counter, and an output pulse is obtained fromcontacts secured to each of the two intersecting ribs when a gamma rayis incident at the intersection.

The contact on each horizontal rib 1 (as oriented in the drawing) isconnected to a separate preamplifier 3 and on each vertical rib 2 to aseparate preamplifier 4.

. Each preamplifier 3 or 4 is followed by a main amplifier and pulseamplitude discriminator 5 or 6 respectively which delivers an outputpulse if the input pulse is above a predetermined amplitude.

The outputs from the eight discriminators 5 are fed to a common binaryencoder 7, and from the eight discriminators 6 to a common binaryencoder 8. Each encoder operates in a manner familiar to those skilledin the art and comprises a plurality of NOR gates to which thediscriminator outputs are connected, the encoder having three outputterminals and the presence of appropriate outputs thereat representingthe digits 0 7. Thus the binary number represented by the output of theencoder 7 is characteristic of the rib 1 at whose intersection a gammaray was incident and similarly the binary number represented by theoutput of encoder 8 is characteristic of the rib 2 at whose intersectiona gamma ray was incident, so that the coincident outputs from the twoencoders 7 and 8 characterise the particular intersection.

The outputs from encoders 7 and 8 are fed to conventionaldigital-to-analogue converters 9 and 10 respectively, which generatedigital-valued output pulses, of duration one microsecond, whoseamplitude is proportional to the input binary number. These pulses areamplified by amplifiers 11 and 12 and applied to the X and Y deflectionplates respectively of a cathode ray tube 14 having a luminescent screen14'. The electron gun of the tube 14 is biassed so that the electronbeam is normally cut off, and the screen 14 is arranged to be excited toluminescence by applying to the electron gun, via circuit 13, uniformbrightening pulses generated by the encoder 7 synchronously with thedigitalvalued pulses; each brightening pulse thus corresponds to theincidence of a gamma ray on one of the intersections between the ribs 1and 2 and for each such pulse there is produced on the screen 14 abright spot at a position corresponding to the intersection where thegamma ray was incident. The sort of pattern obtained from a plurality ofgamma rays incident on the detector, in the absence of modulation, isthus as shown at 15, viz a rectangular array of superimposed spots. Whenthe screen 14 is photographed with a relatively long exposure, thebrightness of the spots on the photograph depends on the number ofsuperimposed spots occurring at each position during the exposure. Theusual adjustable DC shift voltages for adjusting the position of thedisplay on the screen 14' are omitted for clarity.

Such a form of display is difficult to interpret, as already discussed.In accordance with the present invention the digital-valued pulses aremodulated in amplitude to produce deflection pulses for application tothe tube 14 by applying to the inputs of amplifiers 11 and 12, inaddition to the outputs from converters 9 and 10, noise signals derivedfrom a noise generator 16 via capacitors l7 and 18 respectively. Theamplitudes of the applied noise signals are adjustable by means ofvariable resistors 19 and 20.

The effect obtainable is illustrated in FIG. 2. Provided the noisesignal amplitudes are suitably adjusted, the spots of FIG. 1 can beexpanded to form the abutting squares 21 of FIG. 2. Increased ease ofinterpretation is obtained even if the squares are notmade to abut, butit is preferred that they should do so. Eight adjoining squares in FIG.2 are shown shaded more heavily for clarity to represent the position ofa radioactive concentration in the body, although in practice theresulting photograph would show them brighter than the remainder.

It is possible, instead of using noise, to modulate the digital-valuedpulses with a fast sawtooth, but this gives the squares sharply definededges and makes abutting difficult. By contrast, when using Gaussiannoise it can be arranged that the half-amplitude value of the noisecurve corresponds to a point located approximately half-way between theunmodulated positions. Adjacent areas therefore overlap in such a way asto add up to a constant density of spots for an equal number of spotsper position. This effect is illustrated in FIG. 3 where the lines 22and 23 represent adjacent unmodulated positions and the Gaussian curves(which show the probability of the amplitude of the noise at any instantplotted against its amplitude about a DC. level) cross at thehalf-amplitude level 24 located on the line 25 midway between lines 22and 23. The constant sum is represented by line 26.

A further advantage of using Gaussian noise in the gamma cameraapplication, rather than a repetitive waveform, is that the distributionof illumination over each illuminated area thereby matches approximatelythe response of the detector. This is because the detector is used witha collimator (not shown) having rectangular channels which register withthe rib intersections. Since the gamma rays may approach the collimatorat various angles of incidence, not necessarily parallel to the channelaxes, it follows that the detector receives more gamma rays from pointson the body lying on the axes of the channels than on their peripheries,rays from the latter tending to be absorbed in the channel walls to agreater degree.

The noise generator 16 incorporates two noise sources which respectivelyproduce the noise signals fed to the amplifiers 11 and 12; each noisesource utilises the noise current of a Zener diode, e.g. of type CV7142, and comprises a transistor amplifier in which the noise isamplified and whose upper cut-off frequency is reduced by conventionalfeedback means to about kHz so that no appreciable broadening of thespots, as opposed to modulation of their positions, is produced. If thenoise voltage changes appreciably during a deflection pulse, i.e., ifits high-frequency content is too great relative to the pulse duration,the tendency is to broaden superimposed spots instead of distributingnonbroadened spots over the area with a Gaussian probability. Thepreferred effect may be compared with the reproduction of pictures innewspapers, where shading is effected by varying the spacing of uniformblack dots.

For each noise source the noise output is rectified and compared with apredetermined voltage in a difference amplifier, the output of thelatter being connected to control the DC. current through the Zenerdiode and thereby maintain a constant noise output level from thesource.

The brightness of the spot produced by each brightening pulse ispreferably made less than will cause a single spot to saturate the filmbeing used to photograph the screen. It may be arranged, for example,that, say, five superimposed spots are required to produce saturation.This allows superimposed spots to increase the brightness of theresulting photograph, thus enabling more information on distributionbetween areas to be obtained from the photograph.

Normally the exposure time used is so long relative to the count-ratethat the individual spots are not discernible in the resultingphotograph, but it is not essential to operate the invention in thisway.

For a rectangular array of positions as shown, the modulation in the Xand Y directions will normally be of equal amplitude to produce squareilluminated areas, but for other applications this may not be the case.

Although described particularly with reference to its use in a gammacamera, a display system according to the invention is applicable to thedisplay of digital information from other sources, usually but notessentially resulting from events occurring randomly in time.

We claim:

1. A cathode ray tube display system wherein said cathode ray tubeincludes a luminescent screen which may be excited to luminescence by anelectron beam generated within the tube comprising means for modulatingsaid electron beam by means of a series of uniform brightening pulseswhich excite said screen to luminescence, said brightening pulsesrepresenting individual items of information of different kinds; andmeans for deflecting said electron beam by means of deflection pulsessynchronous with said brightening pulses, said deflecting meanscomprising means for generating digital-valued pulses having valuesselected according to the kinds of item represented by said brighteningpulses and corresponding to an array of discrete positions on saidscreen, and means for deriving said deflection pulses by relatively lowfrequency amplitude modulation of said digital-valued pulses so that foreach position of said array the excitation of said screen is distributedover an appreciable portion of the screen.

2. A display system according to claim 1, in which said amplitudemodulation comprises Gaussian noise.

3. A display system according to claim 2, in which said amplitudemodulation is adjustable so that the ex cited portions of said screencorresponding to adjacent positions of said array overlap with half peaknoise amplitude corresponding to a point approximately midway betweenthe adjacent positions.

4. A display system according to claim 1, in which the rate-of-change ofsaid amplitude modulation is small relative to the duration of saidpulses.

5. A display system according to claim 1, in which said means forgenerating digital-valued pulses is operative to generate a pair ofdigital-valued pulses corresponding to each brightening pulse, and saidmeans for deriving said deflection pulses is operative to derive fromeach pair of digital-valued pulses a pair of deflection pulses whichrespectively deflect said electron beam in orthogonal directions,whereby said array of positions is two-dimensional.

6. A gamma camera comprising a display system according to claim 5, atwo-dimensional array of gamma ray counters, and means for generatingone of said brightening pulses in response to the incidence of a gammamy on any one of said counters, said means for generating digital-valuedpulses being operative so that the values of the pair of digital-valuedpulses corresponding to said one of said brightening pulses characterisethe position of said one of said counters in said array of counters.

7. A display system according to claim 1 in which said amplitudemodulation comprises a saw-tooth wave form.

1. A cathode ray tube display system wherein said cathode ray tubeincludes a luminescent screen which may be excited to luminescence by anelectron beam generated within the tube comprising means for modulatingsaid electron beam by means of a series of uniform brightening pulseswhich excite said screen to luminescence, said brightening pulsesrepresenting individual items of information of different kinds; andmeans for deflecting said electron beam by means of deflection pulsessynchronous with said brightening pulses, said deflecting meanscomprising means for generating digital-valued pulses having valuesselected according to the kinds of item represented by said brighteningpulses and corresponding to an array of discrete positions on saidscreen, and means for deriving said deflection pulses by relatively lowfrequency amplitude modulation of said digitalvalued pulses so that foreach position of said array the excitation of said screen is distributedover an appreciable portion of the screen.
 2. A display system accordingto claim 1, in which said amplitude modulation comprises Gaussian noise.3. A display system according to claim 2, in which said amplitudemodulation is adjustable so that the excited portions of said screencorresponding to adjacent positions of said array overlap with half peaknoise amplitude corresponding to a point approxiMately midway betweenthe adjacent positions.
 4. A display system according to claim 1, inwhich the rate-of-change of said amplitude modulation is small relativeto the duration of said pulses.
 5. A display system according to claim1, in which said means for generating digital-valued pulses is operativeto generate a pair of digital-valued pulses corresponding to eachbrightening pulse, and said means for deriving said deflection pulses isoperative to derive from each pair of digital-valued pulses a pair ofdeflection pulses which respectively deflect said electron beam inorthogonal directions, whereby said array of positions istwo-dimensional.
 6. A gamma camera comprising a display system accordingto claim 5, a two-dimensional array of gamma ray counters, and means forgenerating one of said brightening pulses in response to the incidenceof a gamma ray on any one of said counters, said means for generatingdigital-valued pulses being operative so that the values of the pair ofdigital-valued pulses corresponding to said one of said brighteningpulses characterise the position of said one of said counters in saidarray of counters.
 7. A display system according to claim 1 in whichsaid amplitude modulation comprises a saw-tooth wave form.